The present invention relates generally to sensors for detecting a ground fault current, and particularly to a multi-phase Hall effect ground fault current sensor for use in multi-phase motor starter applications.
Ground fault current sensor systems have many uses and applications for detecting ground fault currents in multi-phase power systems. For example, ground fault current sensors can be utilized in multi-phase motor starter applications.
A three-phase induction motor starter typically consists of a contactor, circuit breaker, and an overload relay. In an exemplary embodiment, power to an electric motor is switched via a three-pole contactor with short circuit protection provided by a circuit breaker or fuses. Motor windings are protected from excessive heating through the use of bi-metal or electronic overload relays. In high impedance applications, ground fault detection and current interruption components are used to protect other electrical and mechanical components, e.g. wiring within the system. Some overload relays provide motor winding overload protection as well as ground fault monitoring and protection.
Common approaches currently utilized for sensing ground fault currents in multi-phase motor starter applications utilize either a core balanced toroid or a vector sum of three separate current sensors, sometimes referred to as a phase summation ground fault method. Each of these methods has certain disadvantages overcome by the present invention.
In the core balanced toroid approach, a core balanced toroid is constructed from a donut-shaped, tape wound core that is spirally wound with a secondary winding. Three power leads for each of the three motor phases are passed through the center of the toroid. Because the currents of a balanced three-phase power supply are 120xc2x0 out of phase and of equal magnitude, all currents cancel. This results in a secondary winding reading or current of zero. If, on the other hand, current is leaking out of the system in the form of a ground fault, all three currents no longer cancel and a current is induced in the secondary winding. The ground fault current approximately equals the product of the secondary current multiplied by the number of secondary turns in the secondary winding.
The second approach, i.e., the phase summation ground fault method, utilizes separate current sensors for each phase. The separate current sensors may be current transformers or Hall effect sensors. The signals from each of these current sensors are connected to a summation circuit or are vectorally added with the help of a microprocessor or other microelectronics. If the sum of the currents from each current sensor does not equal zero, then a ground fault may exist somewhere in the system.
Each of these approaches has certain disadvantages. Core balanced toroids, for example, typically are large and bulky. A toroid for use with 500 amp currents may have a diameter on the order of 3 to 6 inches, requiring a larger than desirable package for use in applications, such as motor starter application. Additionally, the secondary winding adds cost, and a resistive load must be added. Furthermore, core balanced toroids may have limited operational range and limited accuracy due to offset errors from high balanced currents and local core saturation effects. The core balanced toroid system also may have limited operational frequency range and/or frequency response characteristics. Further, core balanced toroids are not amenable to the use with a gapped core, and thus they can have linearity problems due to material permeability variations.
In the phase summation ground fault approach, each single current transformer has some error in current measurement accuracy. This leads to a combined error from each of the separate sensors because the signals from the independent sensors are added. Because of the additive error, it can be difficult to discern whether a signal is actually a ground fault or merely a cumulative error. Additionally, the turns ratio of a current transformer is selected to optimize the current transformer performance at a specific current level. If the level of ground fault current is significantly lower than this current, it is very difficult to distinguish ground fault currents from electrical noise.
The present invention addresses the various drawbacks of existing systems and methods for detecting ground fault currents in multi-phase systems.
The present invention features a ground fault current sensor system for use in multi-phase motor starter applications. The system includes a rectangular core and a magnetic flux sensor. The rectangular core is formed from a plurality of laminations and includes a rectangular opening through which multiple conductors are allowed to pass through. The magnetic flux sensor preferably is a Hall generator disposed in a gap formed in the rectangular core. The sensor is oriented to detect changes in the magnetic flux that are indicative of a ground fault current.
According to another aspect of the invention, a ground fault current sensor system is provided for use in multi-phase power applications. The system includes a core and a magnetic flux sensor. The core includes a conductor opening for receiving three-phase conductors therethrough. Additionally, the core includes a gap disposed therethrough proximate the conductor opening. The magnetic flux sensor, e.g. Hall generator, is located in the gap. This sensor is configured to output a signal upon experiencing a magnetic flux resulting from unbalanced current flow in the three-phase conductors.
According to another aspect of the present invention, a method is provided for detecting ground fault currents. The method includes providing a plurality of conductors for supplying multi-phase power. The method also includes substantially surrounding the plurality of conductors with a gapped core, preferably formed of multiple rectangular laminations. The method further includes disposing a sensor in a gap of the gapped core to detect changes in magnetic flux that result from unbalanced current flow in the plurality of conductors. The core preferably is formed of a high permeability material and grounded for electrical noise immunity.